Monkey corpses are what is left after killing a level 12 Karamjan monkey. The monkeys at the Ardougne Zoo drop them as well. Until recently the monkey had to be killed with ranged, magic, or a halberd, now it can be killed with any weapon to retrieve a corpse.
If a player attempts to use a knife on it before starting the below quests, they will receive a message saying: "You don't know how to skin a monkey...yet."